In ophthalmology, the technique of using a photosensitizer and electromagnetic radiation to change the biomechanical and biochemical properties of tissue, e.g., the cornea, for therapeutic purposes has been known for more than 10 years.
The human eyeball is bounded by the corneosclera. Due to internal eye pressure, the corneosclera, which contains collagen, has an approximately spherical shape. In the posterior eyeball region, the corneosclera consists of white sclera. The cornea, which is translucent to visible light, is situated in the anterior region.
Deformations of the corneosclera can cause ametropia. For example, axial myopia, a type of myopia, can result from a cornea and/or sclera longitudinal expansion of the eyeball. An ellipsoidal shaped corneal surface can cause a form of astigmatism or other high order aberration, which is called “irregular corneal curvature”. Another defect of the cornea can be caused by progressive and irregular changes in corneal shape. This is typically known as ectasia. These ectatic changes are typically marked by corneal thinning and an increase in the anterior and/or posterior curvatures of the cornea, and often lead to high levels of myopia and astigmatism. The most common form of ectasia is keratoconus. In keratoconus, a pathological softening of the cornea leads to a progressive thinning and cone-shaped deformation of the cornea. As the bulging increases, the cornea becomes typically thinner below the center. It can fracture and become scarred, which can permanently reduce visual acuity. In these conditions, the corneal stroma is structurally weakened and biomechanically unstable.
Corneal crosslinking (which is also often referred to as cross-linking, corneal collagen crosslinking or corneal collagen cross-linking) is a technique which uses ultraviolet (UV) light or light in the blue spectrum and a photosensitizer to strengthen chemical bonds in the cornea and to thereby increase the corneal stiffness. The stiffening effect results from UV radiation of the photosensitizer. By means of the UV radiation, the photosensitizer is activated to cause corneal crosslinking. Corneal crosslinking involves the cross-linking of collagen fibres. UV light is used to bring about the crosslinking in the cornea, which results in the cornea becoming stronger and more rigid. The UV light that the cornea absorbs during the treatment causes crosslinking. The crosslinking will take place over a few days after the irradiation with the UV light.
In short, corneal cross-linking may be regarded as the process of placing a photosensitizer on or into the cornea followed by exposure to UV light, in order to stiffen the cornea.
In known techniques, the corneal epithelium is at least partially removed to introduce riboflavin as one example of a photosensitizer into the cornea, because the epithelium hinders the riboflavin from penetrating the cornea by acting as a barrier to the diffusion of the riboflavin molecules into the cornea. More recently, it has been proposed to create channels in the cornea by means of a laser device, to introduce riboflavin into the created channels and to irradiate the introduced riboflavin by means of a suitable additional UV light source. This requires that either the additional UV light source is moved to the patient or the patient is moved to the additional UV light source.